Its been an idea I've been pondering recently. Oversaturation of a game series and what it leads to... lack of innovation, cut n' paste ideas and, sadly, ginormous sales figures.
A few examples are Assassin's Creed, Call of Duty, Battlefield...
All of these series have many things in common. they tend to have as many games released as they have years of existence, if not more (in AC's case, 10+ in six years), add very, VERY little to each new iteration, cut and paste many ideas, use the same engine multiple times, have yearly releases ranging from October to November AND, sadly, sell millions.
Meanwhile, we have games series like Final Fantasy (talkin' main series, no spin-offs), Metal Gear Solid, Grand Theft Auto and others that have game releases every 3-5 years... each new title brings many new ideas to both the universe and genre, they build on prior games, utilize new engines and have fair amounts of innovation.
tl;dr Oversaturation of a game series causes it to stagnate, yet, they sell millions to people waiting in line. It causes game developers to not leave their bubble of safety and make a sure thing that'll sell, while people continue to support both the game series and this behavior which leads to lack of innovation.
And, its a sad thing to not see developers, publishers or companies not take risks. If they didn't, we wouldn't see games like LittleBigPlanet, Portal and many others come to fruition.